1. Field of the Invention
The present invention relates to a liquid crystal device and a method of driving the same, and more particularly to a liquid crystal device and a method of driving the same capable of suppressing high intensity lines produced while driving the device in units of blocks by using TFT's (thin film transistors) as switching elements.
2 Related Background Art
As shown in FIG. 3, in a conventional method of driving a liquid crystal panel having a TFT active matrix circuit, internal video signal lines of a display panel 1 are divided into a plurality of blocks. A matrix circuit 2 is provided for matrix-connection between the internal video signal lines of each block and external video signal lines having the same number of lines as the former lines. Sample/hold switching elements constructed of a B-TFT (block dividing TFT) array 3 are interposed on the respective internal video lines between the matrix circuit 2 and the display panel 1. Control signals are supplied to the switching elements of each block to drive the display panel in time division using one horizontal period (lH) as a reversal period.
FIG. 4 shows a detailed connection diagram of FIG. 3, wherein external video signal lines Dl, D2, . . . , Dm are divided into m internal video signal lines Sl, S2, . . . , Sm per one block by the matrix circuit 2. In case of k blocks, the total number of video signal lines is m.times.k. Each of the internal video signal lines Sl, S2, . . . , Sm is grounded via a hold capacitor 10. Switching elements 11 interposed between the capacitor and the matrix circuit are driven in time division by respective block division gate drivers Bl, B2, . . . , Bk to output video signals to pixels.
When a liquid crystal panel constructed as above is driven using one horizontal period (lH) as a reversal period, a charge shift phenomenon of a so-called charge sharing effect occurs at the intersection between divided blocks, e.g., between lines Sm and Sl of FIG. 4, due to the capacitance between source lines of B-TFT's. As a result, .DELTA.V is superposed on the video signal of line Sm so that a video signal having a larger voltage amplitude than the original video signal is outputted (with opposing electrode 12 being grounded).
FIG. 5 illustrates the principle of the charge sharing effect, and FIG. 6 is a timing chart showing the charge sharing effect. In FIG. 5, a central broken line indicates the intersection between blocks, the block at the left of the line being called block 1 and that at the right being called block 2. The last signal line Sm of block 1 is driven by the output signal from the last source line Dm and the drive voltage Bl for the block division TFT's of block 1. The first signal line Sl of block 2 is driven by the output signal from the first source line Dl and the drive voltage B2 for the block division TFT's of block 2, source line capacitance Cm and Cl as seen from source terminal side of the block division TFT's, correspond to the video signal hold capacitor C. Interline capacitance Css producing .DELTA.V appears between the source lines. Referring now to FIG. 6, when a gate pulse is applied to line Bl, a video signal on line Dm is transferred to line Sm via the B-TFT to charge the source line capacitor Cm. After charging the source lines of block 1 to which the capacitor Cm belongs is completed, another gate pulse is applied to line B2 to thereby charge the source lines including line Sl of block 2. In this case, the charging waveforms on lines Sm and Sl at the intersection between the two blocks change as shown in FIG. 6. Particularly, .DELTA.V shown by oblique lines is superposed on line Sm and its video signal becomes larger in amplitude than its original, while the video signal on line Sl changes at the start of reversal as shown by oblique lines. Such phenomenon results from the charge sharing effect of the source interline capacitance Css between the capacitors Cm and Cl. The relationship between .DELTA.V and V is approximately defined by the following formula:
.DELTA.V .apprxeq.Css/(C+Css).multidot.V(v)
(C=Cm.apprxeq.Cl)
If a liquid crystal display panel as above is driven without any correction, the last lines Sm of the blocks are highly brightened so that it is quite unsuitable for a display device.